Device and method to control yarn tension and yarn feeder

ABSTRACT

A device for controlling the tension of a yarn includes a gauge measuring tension of the yarn and optionally a brake driven by the tension gauge to vary braking of the yarn. The gauge comprises electromagnetic control means having a moveable control element subject to a force dependent on the tension of the yarn, and a control circuit. The latter circuit supplies the electromagnetic control means with a current so that the moveable control element remains virtually still in a given position, during variations of the yarn tension. This current is indicative of the tension of the yarn and is measured to obtain a measure of the yarn tension. The device can be associated with a yarn feeder having a drum and moveable eyelet coaxial with the drum.

FIELD OF THE INVENTION

This invention relates to a device for controlling tension of a runningyarn subjected to rapidly varying stresses to a yarn feeder for feedingyarn to a weaving machine, and to a method for measuring the tension ofa yarn at the outlet of a yarn feeder.

BACKGROUND OF THE INVENTION

Controlling yarn tension is very important in order to achieve maximumeffectiveness in many weaving operations, such as insertion of the weftyarn in a high-performance loom, like the modern shuttleless looms, orfeeding of a group of yarns to a warping machine, or still otheroperations. In all these activities, the way in which the yarn is drawnoff is such that yarn tension may vary considerably. A particular caseis insertion of weft yarn in a shuttleless loom, where the yarn is fedin at extremely high and extremely variable speed, causing tension peaksin the yarn that may even result in the yarn breaking.

Mechanical devices are known which control yarn braking in relation toyarn tension, so as to maintain a substantially constant tension in theyarn, far from very high values and exempt from sharp variations. Theseare equipped with brakes that react mechanically to the tension existingat any moment in the yarn, so that the braking force exerted by thebrake on the yarn decreases as the tension increases and, conversely.Braking is mechanically modulated in relation to the yarn tension value.Mechanical devices generally have a slow response time in detectingvariations of tension occurring at high frequency. These drawbacksderive from the fact that the yarn tension is detected to modulatebraking by using mechanical parts engaging with the yarn and that areprone to move during the yarn tension variations. Though of very smallmass, these mechanical parts nevertheless have an inertia and anelasticity that are such that, they interact dynamically with the yarnexchanging kinetic and elastic energy with it, especially in cases of asudden variation in yarn tension. These dynamic variations slow down theresponse and diminish the efficaciousness of these devices to controlthe brake and, as a result, the yarn tension. Moreover, the dynamicinteraction of mechanical parts and yarn has an unsettling effect onexisting yarn status, and may thus considerably modify yarn tension,especially when the tension varies extremely rapidly and the dynamicinteractions between these mechanical parts and the yarn occurcontinuously. These known devices are, in addition to their slow speedof reaction to tension variations, never completely passive with respectto the yarn in detecting its tension and always alter its value to acertain extent, thereby reducing efficaciousness of braking modulation.Furthermore, these known devices do not measure the absolute or numericvalue of yarn tension but are limited to detecting relative variationsof tension in order to control yarn braking.

As known devices do not possess a feature whereby the effective tensionvalue is visualised this prevents these devices from being setaccurately and permits only an empirical regulation (generally performed`by feel` by an operator).

U.S. Pat. No. 5,316,051 shows a truncated cone-shaped cap fitted on thefront end of the drum of a yarn feeder to press directly on the yarncoming from the reserve and passing between cap and drum. This brakeexerts a braking force on the yarn that is responsive to the tension inthe yarn when exiting from the brake itself. In fact, this exiting yarntension gives rise to a tension component oriented axially and apt toact retroactively on the cap for being subtracted from the braking forceexerted by the cap on the yarn. The axial tension component causes areduction of the braking force. Therefore, if yarn tension tends torise, the brake reduces its braking force proportionally. This brakereduces yarn tension peaks and smoothes the tension pattern, but cannotcompletely cease from braking on the yarn, i.e. the yarn can never bedisengaged from the brake completely to reduce yarn tension as much aspossible. This brake cannot entirely nullify the tension at the brakeoutlet. The yarn always remains engaged with and is thus braked by thecap.

Devices are known to control yarn tension that are used on a yarnfeeder, and include yarn tension gauge attached to the eyelet coaxialwith the feeder drum. Tension measurement is obtained by measuring thevariation of parameters due to axial displacement of the eyelet, becauseof the force dependent on yarn tension. The eyelet is thus left free tomove in the axial direction under the action of this force. Thesedevices have the tendency to exchange kinetic and/or elastic energy withthe yarn, thus leading to the risk of the eyelet starting to vibrate.These inconveniences mean that tension measurement is generally affectedby error and is thoroughly unreliable when yarn is drawn off from thedrum at extremely high and rapidly varying speed.

One object of this invention is to provide a yarn tension control devicehaving a very high response speed and high precision in measuring yarntension.

A further task of the present invention is to provide a yarn tensioncontrol device that visualises the effective tension values, so that thelatter may be of use to an operator for convenient setting of thedesired tension threshold values and also make it possible tosubsequently control deviation in time of the tension from the desiredvalues, so that action may be taken if the deviations are found to beanomalous.

Another object of this invention is to provide a yarn tension controldevice which, as well as measuring yarn tension, also has a very highresponse speed and high precision in varying braking force on the yarn,so as to intervene in an effective and timely manner to correct yarntension and maintain it substantially within a programmed pattern,especially when the yarn is subjected to stresses varying extremelyrapidly while it proceeds along its path and which is also able, asrequired, to disengage from the yarn and thus completely annul brakingon the latter.

According to another aspect of the invention, the movable guide elementengaging the yarn is associated with elastic yarn retracting means,which can be regulated manually and which are arranged between themovable guide element and the yarn tension sensor means in order toretract the yarn and keep it constantly under tension, preventing itfrom becoming slack.

According to another aspect of the invention, the motor of the yarntension sensor means may be used for positive driving of the guideelement so as to retract and tension the yarn in cases where it tends tobecome slack as, for example, at the end of a cycle of weft yarninsertion in a shuttleless loom.

The device of the invention is particularly advantageous on shuttlelesslooms due to its capacity to control weft yarn braking during theinsertion stage as a function of the tension effectively acting in theyarn and not, as is the case in known systems, in a way dependent on orsynchronised with the loom cycle. The inventive concept permits thecontrol of yarn tension at the outlet of a yarn feeder, during thefeeding of yarn at high speed into a weaving machine. The yarn passesthrough an eyelet at high speed and exerts a force on the eyelet that isdependent on the tension of the yarn.

The device and method offer considerable advantages, such as that of notaltering the path normally followed by the yarn in the yarn feeder as itunwinds and that of being able to obtain a yarn tension measurement froma tension component apt to assume a significant value, i.e. similar tothat of actual tension of the yarn. In fact, the device of the inventionutilises to advantage the deviation that the yarn undergoes in passingthrough the eyelet coaxial with the drum on its way to the weavingmachine. This arrangement of the eyelet with respect to the drum is suchas to produce a very high angle of deviation of the yarn, incorrespondence with the eyelet, as the yarn reaches the eyelet along apath that is radial with respect to the drum, coming from its outercylindrical surface, whereon the yarn reserve is located. The effect ofsaid high deviation is that the force the yarn exerts on the eyelet andwhich is proportional to both the tension to be measured and to thedeviation undergone by the yarn, assumes a very significant value,similar to that of the tension and apt to permit a precise and reliablemeasure of the said tension.

A preferred embodiment of the invention relates to a yarn tensioncontrol device including a guide element which engages the yarn and issubject to a force proportional to the tension of the yarn, and a yarntension sensor means which detects the tension in the yarn through theguide element. The yarn tension sensor means includes a control circuit,electromagnetic control means to which the control circuit suppliescurrent and which is provided with a moveable control element operableelectromagnetically and a position sensor. The moveable control elementmoves along with the moveable guide element, and the position sensoremits a signal indicative of displacements of the moveable controlelement from a determined measuring position and corresponding todisplacements of the guide element caused by said force proportional tothe tension of yarn. The control circuit supplies the electromagneticcontrol means with current to generate a magnetic control force for thecontrol element for maintaining same substantially motionless in thedetermined measuring position during variations of tension of yarn. Thiscurrent supplied to the electromagnetic control means is indicative ofthe yarn tension, and the current is measured by the tension sensormeans to generate a signal indicative of the tension.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a device for controlling yarn tension;

FIG. 2 is an electrical block diagram of the device of FIG. 1;

FIG. 3 is a perspective view of a variant of the device of FIG. 1;

FIG. 4 is a top view, partially sectioned, of the variant of FIG. 3;

FIG. 5 is a cross-section taken along the line 5--5 of FIG. 1 or in thevariant of FIG. 3;

FIG. 6 is an enlarged scale view of a detail of FIG. 1;

FIG. 7 is a variant of the brake of FIG. 5;

FIG. 8 is a graph showing the time course of operating parameters of thedevice;

FIG. 9 is a perspective view of the device of FIG. 1 equipped withmechanical coupling means;

FIG. 10 is a variant of some details of FIG. 9;

FIG. 11 is a diagrammatic representation of a first form of a damperuseful on the device of FIG. 1 and the device of FIG. 3;

FIG. 12 depicts another form of construction of the damper of FIG. 11;

FIG. 13 is a variant of the block diagram of FIG. 2;

FIG. 14 is a lateral and partial cross-sectional view of a variant ofthe device of FIG. 1;

FIG. 15 is a front view of the variant of FIG. 14;

FIG. 16 is a flow diagram of one mode of operation of the device of FIG.1, to retract yarn;

FIG. 17 is a longitudinal section of the device associated with a yarnfeeder;

FIG. 18 is a first variant of the device of FIG. 17;

FIG. 19 is a second variant of the device of FIG. 17;

FIG. 20 is a third variant of the device of FIG. 17;

FIG. 21 is a front view in enlarged scale of some details of the variantof FIG. 20;

FIG. 22 is an enlarged scale view of a different embodiment of thevariant of FIG. 20;

FIG. 23 is a variant of the device of FIG. 17 with the addition of abrake;

FIG. 24 is a different construction of the variant with brake of FIG.23;

FIG. 25 is an electrical block diagram of the devices and variants ofFIGS. 17-20 and FIGS. 23-24;

FIG. 26 is a further variant of the device of FIG. 17, with a cap brake;

FIG. 27 is a time pattern of parameters applied to control yarn tensionduring insertion in a loom;

FIG. 28 is an electrical diagram representing some parts of theelectrical block diagrams of FIG. 2, FIG. 13 and FIG. 25 in greaterdetail; and

FIG. 29 is a variant of the electric diagram of FIG. 28.

DETAILED DESCRIPTION

In FIG. 1, a device 30, acts on a yarn 31 which moves along a path in adirection indicated by arrow 32. The yarn is guided by eyelets 33mounted on a fixed structure 34 of the device 30. Yarn tension sensormeans 36 suitable for measuring the yarn tension, (also called "tensiongauge" hereinafter), are provided to act, by means of a guide elementengaging the yarn 31, on a portion of the yarn. The tension sensor means36 includes electronic control means having a movable control elementdriven by an electromagnetic force.

The electromagnetic control means and the relative movable controlelement can be embodied by a direct current motor 37 attached to thestructure 34 and its rotor 40. The latter rotates about a casing of themotor 37 and has a shaft 38 protruding from the casing. The movableguide element includes, for example, a rod 41 which is attached to theshaft 38 and acts on the yarn portion, upon which the yarn tensionsensor means 36 acts based on a rectilinear trajectory between the twoeyelets 33.

A generic deviation of an angle α (FIGS. 3 and 5) produced by the rod 41on the yarn with respect to the rectilinear trajectory automaticallyresults in application of a force F by the yarn 31 on the rod 41. Thevalue of F, assuming negligible friction between yarn 31 and rod 41, isequal to 2Tsin(α), where T is the tension in the yarn. The tension gauge36 also comprises a disk 42 which is integral with the rotor 40 of themotor 37, and which is equipped with a slot 43 (FIG. 6). The slot isaligned with a position sensor 44 and, under these conditions ofalignment, establishes a determined measuring position (hereinafter alsocalled reference position) of rotor 40 and rod 41. In this measuringposition of rod 41, there is a given angular deviation α of the yarn 31.To signal alignment of the slot 43 with the sensor 44, the latter isequipped with optical elements 46 apt to detect each displacement of theslot 43 from the alignment condition, and hence of the rod 41 from themeasuring position.

Other types of sensors, for example, Hall effect or inductive sensorsmay be used to signal displacements of the rod 41 from the measuringposition. This position may correspond to a very low angle of deviationα of the yarn, of only a few degrees, so that the additional tensioninduced in the yarn 31 by its sliding on rod 41 is practicallynegligible.

In FIG. 2 blocks of the diagram refer to elements already described inthe foregoing and are therefore indicated with the same numbers. Acontrol circuit 51 receives from the sensor 44 a signal S indicative ofdisplacements of the disk 42 with respect to the position where it isaligned with sensor 44 and hence also of the displacements of rod 41with respect to its measuring position. On the basis of this signal S,the circuit 51 powers the motor 37 with a current I of intensity anddirection such as to maintain disk 42 substantially motionless in thealignment position, instantaneously balancing the resistive torque thatthe force F applies on shaft 38 through rod 41, whatever the value offorce F and hence of the tension T acting on the yarn 31 which iscalculated as (F=2Tsin(α)).

Generation of oscillations in the current I ought to be avoided wheneverthe current varies rapidly in value. The oscillations can normally beattributed to secondary oscillation phenomena and could, if highlymarked, give rise to instantaneous yarn tension measuring errors. Onthis subject, a particularly advantageous form of construction of thecontrol circuit 51, and to which reference will be made by describingthe variant of FIG. 13, involves the use of the digital controltechnique.

As rod 41, rotor 40 of motor 37 and disk 42 remain practicallymotionless during variations of tension, this prevents their inertia andelasticity from having a negative, unsettling impact on detection oftension T. Rod 41, rotor 40 and disk 42 behave as if they were virtuallystill and free of inertia and elasticity in detecting the tension T ofthe yarn 41. The control circuit 51 can drive motor 37 in a PWD-mode(Power With Modulation) to supply power to the motor 37 in pulsing modeand cause I to vary analogically. When the yarn 31 is not subjected toany tension, rod 41 and rotor 40 are not under stress either, and hencethe current I assumes a null or extremely low value that corresponds toa minimum current threshold for controlling position of rotor 40, whenthere is no torque acting thereon. When there is a tension T in theyarn, the rod 41 is subjected to a force F proportional to the tensionand applies a resistive torque on rotor 40 of motor 37. The resistivetorque is equal to the product of F by its arm with respect to the axisof motor 37, and is balanced by an active torque generated by thecurrent I. As the active torque generated by a direct current motor isproportional to the current with which the motor is supplied, the forceF and hence the tension T in the yarn 31 are proportional to the currentI.

Current I and tension T are proportional for all values of the angle αof deviation of the yarn 31. Assuming T remains constant, current Itends to drop as α decreases, since F also decreases correspondingly.The device supplies a current I indicative of the tension T, even forvery low values of angle α, under 10°, so that sliding of the yarn 31 onthe rod 41 is practically incapable of influencing tension T of the yarn31.

A circuit 52 (FIG. 2) for detecting the current I is connected to theline supplying the motor 37 with current I and has high impedance so asnot to influence the value of I. In turn, circuit 52 generates a signalC, indicative of the current I. Preferably, signal C is processed by afilter circuit 53, to filter signal C of the oscillations and noisetypically inherent in operation of the control circuit 51. The filter 53generates a filtered signal indicative of the current I, which is sentto a tension measuring circuit 54 which measures yarn tension. Themeasuring circuit 54, in turn obtains from this filtered signal theeffective value of yarn tension and to enables visualisation of thisvalue on a display 58. The circuit 54 updates the tension value on thevisualizer 58 with a frequency much lower than the measuring frequencyin order to allow the tension value to have a certain stability on thedisplay 58 and thus to be read with ease.

For calibrating the measuring circuit 54, a potentiometer 55 is providedfor being regulated. The tension value visualised by the indicator 58ought to coincide with a sample value for tension of the yarn 31provided by an external measuring appliance, a dynamometer for example,applied on the yarn 31 to pull it in the direction of the arrow 32 whilepotentiometer 55 is regulated.

A setting circuit 56 receives a yarn tension measuring signal fromcircuit 54 and is connected to a push-button 59, or to a functionallyequivalent element. The push-button 59 may be actuated by the operatorto store a reference value TREF corresponding to a set tension value inthe setting circuit 56. In practice, the operator varies yarn tensionmanually and, on observing a tension value on indicator 58 equal to thevalue that he wishes to set, actuates push-button 59, thereby storingthe TREF value in circuit 56.

FIG. 3 represents a device 60 similar to FIG. 1, in which the tensiongauge 36 uses an electromagnet 61 in place of the direct current motor37. This electromagnet 61 is attached to the fixed structure 34 andcomprises a core 62, a coil 63 wound around the core 62 and an armature64 disposed between two extensions 66 and 67 (FIG. 4) of the core 62.The armature 64 has an end portion which together with the end surfacesof the extensions 66 and 67 defines an air gap 68. The core 63 issupplied with a current I' (FIG. 4) by the control circuit 51 (FIG. 2)and generates a magnetic field apt to be which is conveyed by the core62 to the air gap 68, to cross it and to produce in correspondencetherewith a magnetic force FE suitable to attract the armature 64towards a position corresponding to a state of minimum reluctance of airgap 68. Armature 64 is attached to a shaft 69 rotatably coupled to thefixed structure 34 such that the armature 64 is moved longitudinallywith respect to the end surfaces of extensions 66 and 67.

An example of armature 64 and of extensions 66 and 67 and theirdisposition in defining the geometry of the air gap 68 is illustrated inFIG. 4. Magnetic force FE tends to move the armature 64 towards theinside of the space between extensions 66 and 67, increasing the area ofthe armature sides which face the respective end surfaces of theextensions 66 and 67. Furthermore, this geometry of the air gap 68 issuch as to determine a substantially constant course of force FE for agiven current I' supplied to coil 63, along a fairly wide longitudinaldisplacement of armature 64 with respect to extensions 66 and 67. Thisproperty of air gap 68 depends chiefly on the fact that it does not varysubstantially in its geometry while armature 64 moves, since the sidesof the latter stay at a constant distance from extensions 66 and 67. Theshaft 69 is arranged substantially parallel to the portion of yarn 31 onwhich the tension gauge 36 acts and supports at one end the rod 41 whichengages the yarn 31 to deviate same by the angle α. A position sensor71, of the optical type for example and including an emitter part 72 anda receiver part 73, is aligned with one corner of the armature 64 tosignal each displacement of the latter from a fixed reference positionwith respect to extensions 66 and 67. The reference position is selectedwithin the field of displacement of armature 64 with respect toextensions 66 and 67, within which field the pattern of magnetic forceFE assumes a practically constant value.

In electromagnet 61, the magnetic force FE acting on armature 64 assumesa single direction, which is that of reducing reluctance of the air gap68. Unlike the motor 37 which exerts on its rotor 40 active torqueshaving opposite directions to move the rotor in both directions, themagnetic circuit of electromagnet 61 is usually capable of activatingmagnetic forces suitable to act on armature 64 in a single direction,irrespective of the direction of the current I' in coil 63.

In order to cooperate with the magnetic force FE in keeping armature 64substantially motionless in its reference position, armature 64 isassociated with elastic positioning means, for example, two springs 74.The springs 74 at one end are fastened to posts 76 extending from thestructure 34, and at opposite ends are attached to rod 41. One or bothof these posts is adjusted in the direction of arrow 77 to regulate rod41 in the exact measuring position corresponding to the desired angle ofyarn deviation α from the rectilinear feed trajectory of yarn 31 betweenthe two eyelets 33.

The electrical diagram relative to the device 60 is substantiallyequivalent to the one shown in FIG. 2. Current I, motor 37 and sensor 44in the diagram of FIG. 2 is replaced with current I', electromagnet 61and sensor 71; in additional, the control circuit 51 feeds the currentI' to the electromagnet 61 of device 60 differently with respect to howit feeds current I to the motor 37 of device 30. In device 60, controlcircuit 51 does not need to feed current I' to electromagnet 61 in twoopposing directions. This is unlike the case of device 30, in which thecurrent I may be fed to motor 37 in two different directions in order togenerate deflecting torques apt to act in opposition on rotor 40 to holdit in the measuring position.

Control circuit 51, in response to signals received from sensor 71indicating displacements of the armature 64 from the measuring positiondue to variations of the tension T of the yarn 31, feeds the coil 63with the current I' so that the magnetic force FE activated by thiscurrent I' generates a torque with respect to the shaft 69 sufficient tocounteract the torque produced, again with respect to shaft 69, by theforce F that yarn 31 exerts on the tip of the rod 41.

Regarding activation of the magnetic force FE by the current I' tocounteract the force F, force FE, which follows a substantially constantcourse for a wide field of displacement of armature 64 with respect toits measuring position, may easily be controlled by the current I' torapidly assume a value which counteracts the force F.

The signals used by circuit 51 to provide the current I' are solelythose indicating displacements of armature 64 in the direction oppositethat of the force FE, namely the displacements that can be referreddirectly to variations of the tension T of the yarn 31. Any signals ofdisplacements of armature 64 in the same direction as that of the forceFE are used by the control circuit 51 to cancel the current I', so thatit is only the springs 74 that bring armature 64 back to the measuringposition. Accordingly, the action of springs 74 suffices to make up forthe impossibility of the force FE produced by the electromagnet 61 toact from opposing directions on armature 64, as has already been statedon several occasions.

The magnetic force FE acts in a single direction. The elastic force eachof the springs acting symmetrically from two opposing directions on rod41 co-operate with one another in order to counteract all displacementsof the rod 44 away from the measuring position during variations of thetension T of yarn 31. The equilibrium with respect to the shaft 69 ofthe torques produced by the forces FE and F and by the elastic force ofsprings 74 is constantly restored by varying the current I' in functionof displacements of armature 64 from its reference position and hencerod 41, shaft 69 and armature 64 remain substantially motionless duringvariations of the tension T of the yarn 31. Thus, the current I' isindicative of the tension of the yarn 31 and is measured by themeasuring circuit 54 (FIG. 2) in a way substantially equivalent to thatalready described for the device 30.

A brake 81 (FIG. 1 and FIG. 3) may be coupled with the yarn tensionsensor means 36 to act on a yarn portion before the latter reaches rod41 and consequently its tension is detected by the yarn tension sensormeans 36.

In FIGS. 1 and 5, brake 81 includes two pressure elements 82 and 83,facing and pressing against each other with the yarn 31 interposedbetween them. In one example, the elements 82 and 83 can beplate-shaped. Element 82 is fixed and integral with the structure 34.Element 82 is free to move to vary its distance from element 83 as afunction of cross-sectional size of the yarn 31. Element 82 includes ablock 84 equipped with a slot 86 which allows block 84 to slide along apin 87 of an arm 88 connected to the structure 34.

Pressure elements 82 and 83 are pressed together by way of a magneticcircuit comprising the element 82, a core 91 and a coil 89 wound on thecore 91, wherein the latter are both integral with the structure 34 andare disposed adjacent element 83. Pressure element 82 is of aferromagnetic material, unlike element 83 which is of a non-magneticmaterial, stainless steel for example, and thus element 82 permits theclosing of a loop of magnetic flux 92 on core 91 generated by the coil89 when the latter is supplied with a current If. With core 91, theelement 82 defines two air gaps 93 corresponding to the areas in whichthe distance between the pressure elements 82 and 83 is smallest and inwhich a force of attraction is generated magnetically between the latterelements, constituting the braking force FR effectively exerted on theyarn 31. Attraction members 94 of ferromagnetic material are mounted onplate 82 in correspondence with the air gaps 93 and allow force FR to beincreased.

The push-button 59 (FIG. 2) is used to condition operation of the brake81. Push-button 59 can be activated in two modes or positions, in thefirst of which in addition to storing the TREF value, it also enables abraking circuit 96 of the brake 81 to operate under a modulated brakingcondition. After activating push-button 59 in this first position, theTREF value remains constant and does not undergo any alteration untilthe push-button 59 is activated again in this first position. In thesecond position, push-button 59, by way of setting circuit 56, switchesoperation of the braking circuit 96 from the modulated braking mode to aconstant braking mode, in which brake 81 is powered with the constantcurrent If and, consequently, generates a constant braking force on yarn31.

A comparing circuit 57 receives both the TREF value and a value TISTsupplied by the measuring circuit 54, which corresponds to theinstantaneous yarn tension value, and outputs for the braking circuit 96a signal indicating which of the two values, TREF or TIST, is thehigher. If circuit 57 signals that the TIST value is higher than theTREF value and the braking circuit 96 is set for modulated brakingoperation, the braking circuit 96 automatically reduces either entirelyor in part the current If supplied to brake 81, thereby cancelling or atleast reducing braking on the yarn 31. Conversely, if comparing circuit57 signals that TIST is below the value of TREF, the braking circuit 96starts powering the brake 81 again with the current If, restoring it toits nominal value either instantaneously or progressively and therebyincreasing braking on the yarn 31.

When the braking circuit 96 is adjusted by push-button 59 for constantbraking operation, the result of the comparison made by circuit 57 doesnot signal circuit 96 and the latter continues to exert a constantbraking on the yarn. Circuit 96 can be regulated manually by apotentiometer 97 to set the braking in constant braking mode, or to setthe nominal value of the current If and thus of braking which, inmodulated braking mode, corresponds to the threshold value TREF. In thelatter case, the potentiometer 97 and the push-button 59 may be usedtogether, by first operating on potentiometer 97 to vary yarn brakingmanually and change the tension value visualised on the indicator 58until the value visualised is that of the TREF value desired and then,at this point, by activating push-button 59 to set this value for TREF.

FIG. 7 represents device 101 (variant of brake 81) in which the magneticcircuit is disposed completely on one side with respect to a fixedpressure element or plate 102 of non-magnetic material. The magneticcircuit comprises a pressure element 103, also plate-shaped andconstructed of ferromagnetic material which acts as a movable armature,a core 104 and one or more coils 106 wound on the core 104 and which aresupplied with a current to generate a magnetic flux 107. The core 104and the plate 103 define between them air gaps 108, which are traversedby the flux 107 to generate magnetic forces FM. Unlike the brake 81 ofFIG. 5, the air gaps 108 do not close the magnetic flux through thesurfaces of plates 103 and 102 in contact with the yarn.

Plate 103 is pushed by a spring 109 against plate 102 that is fixed tothe core 104 to exert a pressure on the yarn 31 passing between plates103 and 102. The charge C of the spring 109 on plate 102 can be adjustedmanually, by varying the length of the spring 109 by means of a knob 111equipped with a leg which screws into a support 112. The support 112rotates on the structure 34 in order to move plates 103 and 102 awayfrom each other The movable plate 103 is supported to moveperpendicularly with respect to the fixed plate 102 and also tocounteract the tangential force to which it is subjected by the yarn 31because of the sliding friction thereof on plate 103. For example, plate103 can be equipped with a pin which slides along a corresponding hole(not shown) of the core 104.

In device 101, the magnetic forces FM generated by the flux 107 moveplate 103 away from plate 102 against the action of the charge C ofspring 109 in order to reduce yarn braking. In this case, the magneticforces FM increase and has have a negative action on tension T of theyarn 31 and therefore tension T of the yarn 31 decreases. Brake 101 canalso operate both in constant braking mode, in which braking is fixedand is set by varying the load C through the knob 111, and in modulatedbraking mode in which the coil 106 is supplied with current to generatethe forces FM in response to the comparison of a reference value TREFwith an instantaneous tension value TIST as described for brake 81.

Compared to brake 81 and in the constant braking mode, the brake indevice 101 has the added advantage of not absorbing energy as no currentflows through coil 106. Moreover, should the magnetic forces FM prevailover the force C of spring 109, the plates 103 and 102 would open andthe yarn 31 would be freed completely from their grasp.

This characteristic of being able to open and close the plates withrespect to the yarn 31 may also be achieved on the brake 81 describedearlier simply by providing the latter with a spring (not shown in thedrawings) which exerts on plate 82 a force with a direction oppositethat of the forces FR in such a way that when the latter become null,plate 82 moves away from plate 83. For example, this spring may behoused inside the slot 86 and act between the block 84 supporting plate82 and the pin 87 to make block 84 slide on pin 87 away from the brake81.

Plates 82, 83 and 103, 102 of brake 81 and brake 101 respectively can berigidly associated to vary braking force when the brake 81 and the brake101 work in modulated braking mode. In both brakes, the air gaps 93 and108 are placed adjacent to the surfaces of the plates which are incontact with the yarn. The magnetic force generated in the air gaps actsdirectly on these surfaces and in fact prevents the plates from behavingin an elastic fashion, which could disturb control of the braking forceduring its variations resulting from the corresponding magnetic forcevariations. An elastic behaviour is unfavourable and depends essentiallyon the fact that the plates may bend or become deformed, therebyacquiring elastic energy for increasing their reciprocal pressure. Thiselastic energy is then released by the plates when they reduce theirpressure. In modulated braking mode, this elastic energy reduces thespeed with which braking force is controlled, and this renders brakingmodulation less efficacious. According to the above-mentionedcharacteristic, no parts of the plates are subject to deformation duringbraking force variations nor can they store elastic energy, which iswhat would happen had the plates elastic parts been arranged between theair gaps in which the magnetic force determining braking force isgenerated and the surfaces of the plates that effectively brake the yarnand are in direct contact therewith.

FIG. 8 includes diagrams indicating different yarn tension patternswhich result when the yarn is braked by a brake which, in the modulatedbraking mode described earlier, works in rigid mode, and when the yarnis braked by a brake working in elastic mode.

The unbroken line diagram 113 refers to the brake operating in rigidfashion and shows that when the tension T exceeds a threshold value Tscorresponding to the set tension TREF, and consequently the brakingcurrent If is cancelled or at least reduced, the yarn tension continuesto increase only for a very short time Δt before returning rapidly tobelow the threshold value Ts.

The dashed line diagram 114, regarding the brake that operates inelastic fashion, clearly shows that, after Ts is exceeded, the tensiontends to continue increasing for a time Δt1 considerably greater thanΔt, and has a lengthy excursion above the threshold value Ts beforereturning below it. Therefore, in the case of the rigid brake, thecurrent If will be made to vary with a high frequency (diagram 116 withthe unbroken line), braking force will respond immediately to thecurrent If and as a result a course of tension T will be obtained withminimum deviations from the threshold value Ts. On the contrary, in thecase of the elastic brake, the current If will vary at a much lowerfrequency (diagram 117 with the dashed line), braking force will followcurrent If with a delay time and the deviation of tension T from the setvalue Ts will be greater.

For example, in place of a stepped variation of current If, like thatshown in FIG. 8, where the current If goes instantaneously from a set,nominal value Is to a null value when the threshold tension Ts isexceeded, and vice versa, goes instantaneously from zero to the value Iswhen the tension drops below Ts, a ramp type variation may be achievedwhere the current If is made to vary gradually, both when the thresholdtension Ts is exceeded and when tension drops below the threshold.

Moreover, in brakes 81 and 101 the parts arranged between the surfacesmaking contact with the yarn 31 and the gap in which the magnetic forcesare generated, can be very light. Their lightness, together with theiraptitude to operate in a rigid way without deformations, enables thebrakes 81 and 101 to quickly and exactly control the braking of the yarn31 in response to the commands of the yarn tension sensor means 36. Inthis way, the device of the invention is able to instantly to modify,that is in "real time", the normal course of yarn tension and to compelit to follow a desired course, even when the normal course ischaracterised by fast, frequent and high tension peaks induced bycorresponding quick variations of the yarn advancing speed, as is thecase of weft yarn fed or inserted into a modern shuttleless loom of thegripper, projectile or air jet type.

FIG. 9 shows device 120 which differs from the version 30 of FIG. 1 inthat a mechanical coupling means is provided between the rod 41 and themotor 37 for the purpose of making shaft 38 rotate relative to rotor 40with an angular rotation amplified with respect to that of rod 41.Device 120 accordingly acquires greater speed and precision of responsewith respect to device 30, while at the same time motors 37 of smallersize may be used. The greater speed and precision depend essentially onthe fact that the angular displacements of disk 42 are amplified withrespect to those of rod 41 and therefore the sensor 44 can signal withgreater speed each angular displacement of rod 41 from its referenceposition as a result of variation of tension T of the yarn 31. Inaddition, smaller size motors 37 may be used given that the torqueacting on them is lesser in inverse proportion to the amplification ofthe angular displacements of rod 41. The coupling means comprise twomeshing gears 121 and 122, of which gear 121 has a greater pitchdiameter than that of gear 122 and is attached on a shaft 123 rotatablerelative to the structure, and the gear 122 is attached to the shaft 38of motor 37.

FIG. 10 illustrates mechanical coupling means, constituted by an arm 124divided into two parts 126 and 127, which are attached by one endrespectively to shaft 123 and to shaft 38 of the motor 37 and have theirother two ends connected together inside arm 124 by means of adeformable element 128. The part 126 is longer than part 127 and thusarm 124 transfers, after amplifying it, the angular displacement of rod41 to shaft 38, simultaneously deforming element 128.

To damp possible mechanical vibrations, induced in the rod 41 and therotor 40 by variations of tension, damping means 129 in FIG. 11 areprovided which cooperate with rod 41 and rotor 40 to dampendisplacements thereof. The damping means may comprise a rubber element131 placed between the rod 41 and the shaft 38 of the motor 37.

According to FIG. 12, the damping means 129 comprise a case 132connected to the shaft 38 and bearing the rod 41, hermetically sealedand filled with a viscous fluid 133, and a mass 134 which is free tomove inside the case 132, though the displacements thereof are braked bythe viscous fluid 133 which occupies the space between case and themovable mass 134. The displacements of mass 134 take place later thanthose of the case 132 and the rod 41 because they are damped by thefluid 133 and, jointly by way of reaction, the rotating displacements ofrod 41 and case 132 are also damped.

FIG. 13 illustrates one variant of the electrical block diagram of FIG.2. In this variant, some circuits which in FIG. 2 were separate are nowintegrated in a single circuit or microcontroller 141. The latter isequipped with a computing section (CPU) 142, a memory 143 and ananalogic/digital conversion section (A/D) 144, and receives, through theAND section 144, the values sampled by a sample & hold type circuit 146of the signal C, which is in turn provided to circuit 146 by the currentdetecting circuit 52. On the basis of the sampled values, themicrocontroller 141 commands the braking circuit 96 and the display 58,visualising on the latter the tension value measured for the yarn 31.Furthermore the microcontroller 141 is connected by way of a line 147,whereupon digital signals are transmitted in both directions to aposition control circuit 148 which digitally controls motor 37. Themicrocontroller 141 and the position control circuit 148 are parts of adigital type position control which, has advantages over analogic typecontrols, such as having greater precision and speed and beingpractically insensitive to noise. In the memory 143 of microcontroller141, a program is stored that manages the different operations of thedevice 30 of the invention. This program may carry out a plurality offunctions, such as updating and visualising on display 58 the maximumvalue of tension T measured over a given period of time, the meantension value calculated and other statistical data concerning behaviourof the tension over a given period of time, and still other functions.The values cited above can be either calculated and stored in the memory143 in anticipation of a request from the operator to visualise them, orthey can be visualised automatically at programmed intervals. Themicrocontroller can be conditioned by a RESET push-button 149 to resetthe device 30, causing the above-mentioned program to start again fromthe beginning, and by one or more setting push-buttons 151 which areprovided to set the threshold value Ts for the tension of the yarn 31 ina way similar to the push-button 59 in the case of operation of thedevice in modulated braking mode, or to set the device for constantbraking mode operation.

The described electrical diagram is also applicable for device 60 ofFIG. 3 in which the electromagnet 61 is used in place of the motor 37.FIG. 13 illustrates, close to the motor 37 and by way of alternative tothe latter, electromagnet 61 with the sensor 71. The way in whichcurrent is provided by the position control circuit 148 differs frommotor 37 to electromagnet 61, i.e. bidirectionally in the case of themotor 37 and unidirectionally in the case of the electromagnet 61.

Rod 41, which serves as the guide element for yarn 31 in detectingtension T, can also act as a retractor for the yarn, when it tends toslacken. FIG. 14 shows that an elastic yarn retracting means including aspring 161 is arranged between the rod 41 and the shaft 38 and exert onrod 41 a torque that tends to cause it to rotate in the direction of thearrow 162 (FIG. 15). Therefore this torque acts on rod 41 in a directionopposite that of the force F generated by the yarn 31. The spring 161consists of a series of windings disposed around the shaft 38 and hasone adjustable end attached to an adjustment knob 163 and the other endconnected to a sleeve 164. Sleeve 164 is coaxial with and rotatable onthe shaft 38 and rod 41 is attached to sleeve 164. The knob 163 is usedto regulate the charge of the spring 161 on sleeve 164 by rotating theadjustable end of spring 161 with respect to the shaft 38 so as toprovide its windings with a varying amount of torsion. In order to lockthe knob 163 in numerous adjustment positions, locking means 166 havebeen provided, comprising for example a small ball housed in the knob163 and pushed by a spring to couple with recesses located in the outersurface of the shaft 38 along its circumference.

Whereas the shaft 38 is held motionless in the measuring positiondefined by the alignment between disk 42 and position sensor 44, the rod41, being connected by elastic means to shaft 38 through spring 161, canoscillate during variations of the tension of the yarn 31. To limitthese oscillations and to transmit to shaft 38 a torque that is asindicative as possible of tension of the yarn 31, the sleeve 164 isslidingly mounted on shaft 38 by means of a unidirectional coupling 167and a viscous joint 168. The unidirectional coupling 167 is disposedbetween the sleeve 164 and the viscous joint 168 and renders the rod 41and the sleeve 164 integral with the viscous joint 168 only when the rodrotates in the direction opposite that of the arrow 162, under theaction of the force F dependent on the yarn tension.

The viscous joint 168 connects the unidirectional coupling 167 with theshaft 38 and includes a rotating flange 169 which is closed inside acase 171, and coupled with internal surfaces thereof through a viscousfluid. The viscous fluid fills the spaces between the rotating flange169 and the case 171, and gaskets 172 prevent the viscous fluid fromseeping out of the case 171. The case 171 is fixed to shaft 38 and istherefore subjected to the same torque that the flange 169, whilerotating in viscous manner with respect to the case 171, exerts on it.This torque is, in turn, determined by the force F of the yarn 31 on therod 41 and therefore also by the yarn tension T. By damping therotations of rod 41, the viscous joint 168 ensures that the latterrotates at a substantially constant speed during variations of thetension of the yarn 31 and thus permits a yarn tension measurement to beobtained that is minimally effectively disturbance due to the inertia ofrod 41. Furthermore, the unidirectional coupling 167 leaves the sleeve164 free to rotate with respect to the viscous joint 168 according tothe direction of the arrow 162, namely when the charge exerted by thespring 161 prevails over force F of the yarn tension on rod 41, and rod41 thus rotates to retract and put in tension the slackened yarn.Accordingly, when rod 41 has to act as a yarn retractor, it turnsextremely rapidly and in a way similar to that of known retractorshaving rods that act solely to retract the yarn.

As rod 41 can rotate with respect to shaft 38, the force F actingthereon will vary not only as a function of the tension T of the yarn31, but also as a function of the yarn deviation angle α or, by the sametoken, as a function of the angle θ (FIG. 15) which defines angularposition of the rod 41. For obtaining an exact measurement of thetension that takes into consideration the angular position of rod 41, asensor of angular positions 173 is provided which transmits a signal POSto both the measuring circuit 54 and the microcontroller 141 which isindicative of the various positions of rod 41, in order to conditionmeasurement of the tension of the yarn 31. The position sensor 173 maycomprise an optical group 174 attached to the fixed structure 34, and ablade integral with sleeve 164 in which a plurality of notches 177 areprovided which correspond to different positions of the rod and createoptical interference on group 174 to generate the POS signal.

As an alternative to using elastic means 161 to make rod 41 as a yarnretractor, it is possible to use the measured value of tension T of theyarn 31 to drive the motor 37, so that the motor 37 acts positively onthe yarn 31 by means of rod 41, and exerts a force of preestablishedvalue on the yarn to retract and tension the yarn 31.

FIG. 16 illustrates the operation of the motor 37 as a yarn retractor. Aprogram which produces this form of operation may be stored in thememory 143 of microcontroller 141. Block 181 indicates the nominal stateof motor 37, in which the rotor 40 thereof is held motionless in themeasuring position in order to measure tension of the yarn 31. Thedecision-making block 182 measures if the yarn tension has been keptnull or at least very low for a fixed time TPREF, and if so activatesthe block 183 in which motor 37 acts as a yarn retractor; if not, on theother hand, it continues to maintain motor 37 in the measuring condition181. In block 183, motor 37 generates a torque to apply a preestablishedforce on yarn 31 through rod 41, which consequently rotates and leavesthe measuring position. In block 184, the signal S supplied by theposition sensor 44 is used to detect whether or not rod 41 passesthrough the measuring position again. If so, a return is made to block181 where the motor locks onto and holds the rod motionless in themeasuring position so that tension of the yarn 31 can be measured. Ifnot, the status corresponding to block 183 is maintained, whereby motor37 acts for retracting and tensioning yarn 31.

A typical situation in which tension of the yarn may tend to cancelitself out for a certain time may occur at the end of a cycle ofinsertion of weft yarn. During such time, motor 37 is therefore made tooperate as a yarn retractor, before returning to act as a tension gaugeimmediately at the start of the subsequent insertion cycle, when rod 41is abruptly rotated to by the yarn drawing and consequently the rod 41passes through the measuring position, where it is locked.

The device to control yarn tension can be associated with a yarn feeder190 (FIGS. 17-20), for example, a weft yarn feeder for a shuttlelessloom. The feeder 190 is provided with a drum 191 having an axis 192. Aplurality of windings 196 of the yarn 31 are wound around a drum 191 toform a reserve of yarn. The windings are unwound to feed the yarn 31 toa weaving machine (not shown in the drawings) in the direction indicatedby the arrow 193. The device to control yarn tension is arrangedfrontally of the drum 191, where it is borne, at least partially, by asupport 194 integral with the fixed structure of the feeder 190. Thedevice also included yarn tension measuring means ("tension gauge")which include electromagnetic control means provided with a movablecontrol element.

In the yarn feeder 190, in order to unwind regularly from the reserve ofwindings 196 without creating entanglements or slipping of the windingsor disorderly movements in general, the yarn 31 needs to a certainextent to be held adherent to the drum 191 while it is drawn off fromthe reserve and moves radially with respect to the drum 191 towards theaxis 192. This can be generally achieved by lightly braking the yarn 31against the drum 191 along the trajectory of the yarn immediatelyfollowing the reserve of windings 196. A flexible cap 197 (or similarelement) is arranged against a rounded end of the drum 191 in order tobrake the yarn 31, which passes between drum 191 and cap 197, while itmoves circumferentially along the rounded end as it is drawn off. Thecap 197 is adjustable along the axis 192 so as to vary its pressure ondrum 191 and hence also braking of the yarn 31.

The device to control tension, may include only the yarn tension gauge,or a combination of the tension gauge with a brake, wherein the gaugeand brake cooperate reciprocally to control yarn tension.

In the former case, as the yarn tension measuring means are practicallyunable to influence the yarn tension but are confined to detecting itsvalue, the cap 197 is the only effective braking element and istherefore used to regulate the operating tension with which the yarn 31is fed by the feeder to the weaving machine. This means that, in theembodiments of the device without the brake operating together with theyarn tension measuring means, the cap 197 is normally adjusted alongaxis 192 to exert a more consistent braking on the yarn 31 with respectto the strict minimum amount required to guarantee regular unwinding ofthe yarn 31 from its reserve.

On the contrary, when the device is equipped with the brake associatedwith the tension measuring means, the brake is responsible for definingthe tension at which the yarn 31 is fed to the weaving machine and, inthis case, flexible cap 197 is adjusted to exert on yarn 31 only theminimum braking already stated.

In the device 200 in FIG. 17, the yarn tension gauge 201 is disposedalong the axis 192 and is oriented symmetrically around the axis inorder to convey the yarn 31 to the weaving machine through an outletchannel 202. In the yarn tension gauge 201, the electromagnetic controlmeans and the movable control element are constituted respectively by amagnetic circuit having a structure similar to that of apermanent-magnet linear motor of the type commonly known as "voice coil"and by a coil 203 coaxial with the axis 192 and partially immersed inthe magnetic flux generated by the magnetic circuit. The yarn tensiongauge 201 also includes a sleeve 204 which supports the coil 203 bymeans of a flange 206 and which is arranged coaxially to the axis 192.An eyelet 207 is attached to one end of the sleeve 204 and deflects theyarn 31 towards the axis 192. The eyelet receives yarn 31 radially whilethe latter bends around the eyelet, coming from the cap 197 (FIGS.17-20).

The magnetic circuit generates a magnetic field by means of a magnet208, and defines a ring-shaped air gap 209 which is traversed by themagnetic field. A central part of the coil 203 extends along the gap209. The sleeve 204 may be linked to the fixed part 194, on conditionhowever that it maintains the capacity to move along the axis 192 insuch a way that both the eyelet 207 and the coil 203 move along theaxis. For example, the sleeve 204 may be housed slidingly in a support211 consisting in a ball bearing for axial sliding with low friction. Inturn, eyelet 207 is subjected to an axial force the value of whichdepends on both the tension T in the yarn 31 and also on the angle ofdeviation of the yarn 31 in relation to the eyelet 207.

Taking β (FIG. 17) to indicate the supplementary angle of the yarndeviation angle (the exact angle of deviation is in fact equal to(180°-β)), the axial force FA may be calculated using the formula: FA=2T(cos(β/2))2. As an alternative to fitting slidingly on support 211,sleeve 204 may be linked elastically to the fixed part 194 by diaphragms212 (FIG. 23) which are elastic in the direction of the axis 192 andrigid perpendicularly to the axis, or equivalent elastic elements, thatguide the tube 204 and permit it to move along the axis 192 only.Position sensor 213 emits a signal S1 to a position control circuit 214(FIG. 25) as a function of axial displacements of coil 203 with respectto a given measuring position along the axis 192 and corresponding toidentical axial displacements of the eyelet 207 caused by the axialforce FA. In turn, the control circuit 214 supplies coil 203 with acurrent I1, depending on the signal S1 received from sensor 213.Position sensor 213 may comprise an optical group constituted by anemitter and a receiver cooperating with a blade 216 integral with eyelet207 and coil 203, wherein the blade 216 intercepts radiation emitted bythe emitter in the direction of the receiver in order to generateposition signal S1. The magnetic circuit includes two cores 217 and 218of ferromagnetic material having an axially symmetrical shape withrespect to axis 192 and arranged on both sides of the permanent magnet208, in order to convey the magnetic field produced by the latter acrossthe air gap 209, wherein the magnetic field is apt to generates amagnetic driving force on coil 203 in the direction of the axis 192 whensaid coil 203 is supplied with the current I1.

In operation of device 200, any axial displacement of coil 203 withrespect to its measuring position or, similarly, of eyelet 207 withrespect to a rest position thereof corresponding to the measuringposition, is signalled by sensor 213 to control circuit 214. In turn,the latter to operates operate on the basis of the signals received fromsensor 213 to prevent coil 203 from moving significantly far away fromits measuring position. For this purpose, control circuit 214 sends intocoil 203 the current I1 with a value such as to generate a magneticdriving force which acts on the coil 203 and balances, in the area ofeyelet 207, the axial force FA caused by the yarn.

Accordingly, coil 203 is held virtually immobile in the measuringposition, rendering practically insignificant all exchanges of kineticand elastic energy between yarn 31 and eyelet 207 during variations ofthe force FA and the corresponding variations of the yarn 31 tension.Current I1 thus adapts instantaneously to the value of the force FA andconsequently is indicative of the tension T in the yarn 31.

Moreover, the angle β associated with the yarn deviation at eyelet 207,being close to 90 degrees, defines a force FA (according to the formulaprovided earlier FA=2T(cos(β/2))2) which is of the same order ofmagnitude as the tension T and which, in any case, for equal values ofthe tension T, is much higher than the corresponding force F of FIG. 1(in that case, in fact, F=2Tsin(α) with α being fairly low).Consequently, current I1 will also assume a more significant and highervalue in order to balance force FA, with respect to the value of thecurrent I associated with the force F, thus greatly improving theprecision and reliability of the tension T measurements.

Current I1 supplied to coil 203 is detected by a circuit 219 (FIG. 25)and generates a signal C1 which is sampled by a sampling circuit 221 andthen sent to a microcontroller 222. The microcontroller 222 takes theeffective value of tension T from the sampled C1 signals and displaysthis value on a visualizer 223, in the same way as already described inrelation to the embodiments of FIG. 2 and FIG. 13.

The microcontroller 222 comprises a computing unit (CPU) 224, a memory225 which stores a program planned to control the different operationsof device 200, and an analogic-digital conversion section 226 (A/D)which receives the sampled values of signal C1. Finally, themicrocontroller 222 is connected to a RESET push-button 227, forresetting of the device 200.

In FIG. 18, device 230 includes electromagnetic control means of theyarn tension measuring means 201 constituted by an electromagnet. Theyarn tension measuring means 201 includes a case 231 of metallicmaterial, a winding 232 inside case 231 which is supplied with a currentI2, a sliding bush 233, and a cylindrical armature 234 coaxial with theaxis 192 and supported slidingly along the axis by bush 233. Armature234 includes the movable control element. Eyelet 207 is connected to thecylindrical armature 234 by means of a sleeve 236 which transmits to thelatter each axial displacement of eyelet 207 caused by variations intension of the yarn 31. Also provided are elastic means, which arefunctionally similar to the springs 74 of variant 60 (FIG. 3) describedearlier, constituted by two springs 237. The springs 237 are arrangedbetween the case 231 and the group comprising the eyelet 207, the sleeve236 and the armature 234, and act on this group to hold same elasticallyin a position of equilibrium along the axis 192, in which position thesprings balance each other reciprocally (measuring position). Thetension gauge 201 comprises a Hall effect sensor 238 associated with thearmature to detect axial displacements thereof, with respect to itsmeasuring position due to variations of the tension T of the yarn 31.

Operation of device 230 with an electromagnet differs from that of thedevice 200 in that, in order to keep the armature 234 substantiallymotionless in the measuring position during yarn tension variations, itis not possible to exert opposing magnetic forces on said armature 234.In fact, similarly to device of FIG. 3, the magnetic force generated bythe electromagnet and acting on armature 234 assumes a single directiononly, irrespective of the direction of the current I2 in winding 232.For this reason, the current I2 is supplied to winding 232 in onedirection only, namely that indicated on the right in FIG. 25 withreference to device 230.

Armature 234 is subjected, in a single direction along axis 192, to amagnetic force which opposes force FA and is activated by the current I2as a function of axial displacements of armature 234 which are directedin accordance with the force FA and, in both directions along the axis192, to the elastic forces of springs 237. Accordingly, both themagnetic force activated by the current I2 and the elastic forcessupplied by springs 237 cooperate to maintain armature 234 in itsmeasuring position and permit current I2 to assume a value indicative ofthe tension T of yarn 31.

The yarn tension control device 240 in FIG. 19 includes yarn tensionmeasuring means 201, electromagnetic control means and the correspondingmovable control element constituted by a motor 241 and by the rotorthereof. The motor is powered by a current I3 supplied by controlcircuit 214 (FIG. 25) and is attached to structure 194 of the yarnfeeder 190 by means of a support 242 in a region disposed laterally withrespect to the axis of drum 191. The rotor of the motor 241 is equippedwith a shaft 243 to which one end of a rod 244 is attached. The rod inturn supports on its other end the eyelet 207. Shaft 243 is thus subjectto a torque, the value of which is equal to the product of the force FAacting in correspondence with the eyelet 207 and depending on thetension T of the yarn 31 by the arm of the force FA with respect to theshaft 243. This arm is determined essentially by the length of rod 244.Also provided is a position sensor 246 which cooperates with a blade 245integral with shaft 243 and rod 244 to signal each rotation no matterhow small, of the rotor of motor 241 with respect to a given measuringposition.

In operation, device 240 has the advantage over the previous devices 200and 230 in that it does not create any encumbrance in the region aroundeyelet 207, and consequently permits comfort of operation in thisregion, in manually threading the yarn 31 into the eyelet 207, forexample.

FIG. 20 illustrates a device 250, in which the electromagnetic controlmeans and the associated movable control element are constitutedrespectively by a flat-shaped motor 251 and by a winding thereof 252,also flat in shape. Motor 251 can be classified as a rotary voice coilactuator. Winding 252, which corresponds to the rotary coil, is suppliedwith a current I4 and is supported by a frame 253 which, in turn, ispivotally mounted on a fixed pin 254 to allow the winding 252 to rotatebetween two permanent magnets 256 (FIG. 21), keeping at a constantdistance from the two. The permanent magnets 256 are supported by acasing 257 which is attached to the fixed structure 194 by means of asupport 258 and is constructed of a metal sheet bent into a `U` shape.The permanent magnets 256 generate a magnetic field suitable to interactwith winding 252 supplied with the current I4 so as to generate amagnetic force acting on the latter to rotate it around pin 254. Themagnetic force may cause the flat winding 252 to rotate either clockwiseor counter-clockwise, depending on the direction of the current I4 inwinding 252. Frame 253 is connected to one end of a rod 259, which inturn supports the eyelet 207 at its other end. Finally an opticalposition sensor 261 is placed between the fixed structure 194 and theframe 253 to signal any displacements of winding 252 from a givenmeasuring position, due to corresponding axial displacements of eyelet207 caused by variations of the force FA depending on the tension T ofthe yarn 31.

In FIGS. 20 and 21, the winding 252, the frame 253, the rod 259 and theeyelet 207 form a compact group, comparable to a balancer leverpivotally mounted on pin 254. Consequently, the relationship betweendisplacements of the eyelet 207 and the corresponding displacements ofwinding 252 is equal to the relationship between the respective radialdistances from pin 254. This configuration gives the device optimumsize, for achieving the maximum speed of response in measuring yarntension.

In device 250, an important advantage is that its parts have angulardisplacements, even if very slight, around the measuring position,unlike other embodiments, such as 200, in which the corresponding partshave rectilinear displacements. The parts of device 250 which pivot onpin 254, are subjected to a very low rotary friction genericallydependent on a yarn tension component acting on the eyelet 207. Indevice 200, the sleeve 204 is constrained to move solely along the axis192, causing a low friction to act on sleeve 204 due to sliding thereon.In device 250, the position of the fulcrum along the lever, the sectionsof the arms, the moment of inertia with respect to the fulcrum of thebalancer lever, etc. lead to optimum performance. It does not possessparts arranged concentric with and adjacent to eyelet 207, so that thearea around eyelet 207 is free. The operation of device 250 isessentially the same as described above. The electrical block diagram ofFIG. 25 indicates schematically at the bottom right how the yarn tensioncontrol devices 200, 230, 240 and 250 are interfaced electrically withthe rest of the diagram. Devices 200, 240 and 250 operate in like mannerwhereas the device 230 with electromagnet has provision for supply ofcurrent I2 in a single direction.

FIG. 22 illustrates mounting of frame 253 on support 258, wherein frame253 instead of pivoting on the support 258, is connected thereto bymeans of an elastic plate 262. The plate 262 cooperates elastically withthe magnetic forces acting on winding 252 to maintain the lattersubstantially motionless in its measuring position.

Device 270 illustrated in FIG. 23 includes, in addition to tension gauge201, an electromagnetic type brake 271. Brake 271 cooperates with gauge201 to brake the yarn 31 as a function of the yarn tension measured bygauge 201. The brake 271 is arranged along the feed path of yarn 31,immediately before the tension gauge 201, which gauge 201 thus receivesthe yarn 31 directly from the brake 271. Brake 271 includes two disks272 and 273 which face each other and are coaxial with axis 192 andpress yarn 31 between them. Disk 272 is arranged near the drum 191 anddisk 273 is disposed near the yarn outlet channel 202. Disk 273 is alsoequipped with a central hole 274, through which yarn 31 enters theeyelet 207. The latter, in turn, is disposed inside the hole 274, insuch a way as to guide yarn 31 during the majority of the deviation ofits trajectory towards axis 192. Disk 272 is fixed, whereas disk 273 ismovable axially along a guide 281 formed in the support 194, so as to beable to adapt to the cross-sectional size of yarn 31. A magnetic circuit276, including a coil 277, a core 278, and the disk 273 (constructed offerromagnetic material) operate to press disk 273 on disk 272 (which isin non-magnetic material) to vary instant by instant braking force onthe yarn 31, while it passes between disks 272 and 273.

Brake 271 is controlled by a braking circuit 279 (FIG. 25) and canoperate either in a constant braking mode or in a modulated brakingmode. Provided for this purpose (FIG. 25) are a regulator 282 of thecurrent If sent to brake 271, for manual regulation of the braking forceon yarn 31, and a setting push-button 283 for setting a reference valuefor yarn tension T, at which modulation of brake 271 must be activated,in a way substantially equivalent to that described in relation to FIGS.2 and 13.

FIG. 24 illustrates device 290 (similar to device 270), in which brake271 is replaced by a brake 291, apt to generate a magnetic force whichis subtracted from the mechanical force exerted by a spring 292 to varybraking on the yarn 31 when working in modulated braking mode, similarlyto the way described in relation to the devices 81 and 101. Brake 291includes two disks 293 and 294 pushed against each other by spring 292,and a magnetic circuit co-operates with disk 294 by means of an air gap296 in order to move disks 293 and 294 away from one another. The airgap 296 is traversed by a magnetic field generated by a coil 297 andconveyed towards the air gap 296 by a core 298. The pressure exerted byspring 292 on disks 293 and 294 is manually adjustable by means of athreaded ring-nut 299 which moves along the axis 192, screwing on to afixed part 302 of the device 290. Disk 294 slides axially on a guide 301formed in core 298 in order to adapt to the cross-sectional size of yarn31 and to be able to move away from the fixed disk 293. Disk 294 has acentral hole 303, wherein eyelet 207 is housed of the yarn tension gauge201 (shown in FIGS. 17 and 23).

Brakes 81 and 101, and brakes 271 and 291 have parts arranged betweenthe gap in which the magnetic forces are generated and the surfaces incontact with the yarn, which are characterised by a high mechanicaltransitivity. These parts are advantageously realised to be very lightand to operate rigidly without elastic deformations.

The brakes installed on feeder 190 and responding to the yarn tensionmeasuring means 201 (hereinafter also called yarn tension sensor means201) are not to be limited to the brakes 271 and 291 described earlier.In another embodiment, the brake associated with the tension measuringmeans 201 includes an annular cap brake 300 (FIG. 26) of similar shapeto the cap 197 described earlier and which is arranged on one roundededge of drum 191. The annular cap brake 300 may make use of otherbraking devices superfluous.

Modulation of the braking produced by brake 300 acts on the yarn as soonas it comes out of the reserve of windings 196 on drum 191, and not inthe area of eyelet 207, as in the case of brakes 271 and 291. Tomodulate braking, the annular cap brake 300 is associated withelectromechanical actuating means which drive it and alter its pressureon the rounded edge of drum 191, in response to the signals of the yarntension sensor means 201. The operation and purpose of theseelectromechanical actuating means for driving the annular cap brake 300are substantially similar to those of the electromechanical meansassociated with brakes 271 and 291, i.e. to rapidly vary the braking ofyarn 31, even though the electromechanical actuating means for drivingthe brake 300 are structurally different from the electromechanicalmeans associated with brakes 271 and 291, on account of the differentlocation and the greater braking surface of the annular cap of brake300.

Device 310 in FIG. 26 is a combination of the yarn tension measuringmeans 201 with the annular cap brake 300 resting against a rounded edge304 of one end of drum 191. Cap brake 300 comprises a thin,non-extendible, continuous body 305, that has the shape of a truncatedcone and is symmetrical with respect to axis 192. The truncated conebody 305 is intrinsically flexible when free, but can take on apractically non-deformable configuration when pressed against the drum191 by resilient means 306 which place body 305 under traction. The body305 is designed to remain substantially non-deformable during variationof the load applied on it by resilient means 306 and/or by theelectromechanical actuation means associated with the body 305 itself.

The fact that body 305 fails to bend but locally deforms where the yarnis passing during load variations characterises both the structure andoperation of the body 305 with respect to the cap 197 employed in theembodiments described earlier. For a better understanding of thedifferences between the body 305 and the cap 197, it should beremembered that cap 197 is designed for operation under conditionsentirely different from those of body 305 of brake 300. The latter infact has to be operated in order to modulate braking, whereas cap 197 isarranged so as to exert a constant braking force all during operation.At best, cap 197 may be regulated selectively, prior to operation, inorder to vary and set its braking action. Cap 197 is normallyconstructed in such a way as to bend during regulation along axis 192,so as to vary pressure on drum 191. Note that this flexibility of cap197 does not have a negative effect on its operation, since cap 197maintains a substantially stable configuration during operation. Cap 197may be comprised by a plurality of metallic strips designed to flex andvary their pressure on the drum during manual regulation or, in place ofthe metallic strips, by brushes that are also pliable.

By virtue of its substantially unbending configuration in the operation,the truncated cone shape body 305 can respond extremely rapidly, byvarying its pressure on drum 191, to the commands of theelectromechanical actuating means, which consist for example of aring-shaped magnet 307 fastened to the fixed structure 194 and includingcoil 308 and a ring-shaped core 309. Provided for performing this highresponsiveness is a flange 311 which is fastened to one end of body 305and which is attracted by magnet 307 to reduce pressure of the truncatedcone body 305 on drum 191, that is on yarn 31, in opposition toresilient means 306. Resilient means 306 may be adjusted with respect toframe 194 of yarn feeder 190 by means of a movable support 312, thusvarying the pressure exerted on drum 191. It is also possible toregulate an air gap 315 between magnet 307 and flange 311 in order toset the attraction force of magnet 307 on flange 311.

A control unit 317 receives a position signal S from yarn tensionmeasuring means 201, supplies means 201 with a current IBAL to balancethe forces acting on eyelet 207 caused by yarn tension T in order tokeep the eyelet 207 substantially motionless in a predetermined positionand, as a function of the value of the current IBAL, supplies magnet 307with a current IM to modulate braking on yarn 31.

With the device 310 of FIG. 26, braking force controlled by measuringmeans 201 is subtracted from a braking force imposed manually by way ofresilient means 306. Other schemes may be envisaged, such as the one inwhich the body 305 is positively pressed against yarn 31 byelectromagnetic means controlled by yarn tension measuring means 201and, for example, housed inside drum 191 in front of body 305.

In other possible combinations of yarn tension measuring means 201 withbraking devices, the latter may consist of brakes that produce theirbraking action essentially by deflecting the trajectory of the yarn.These brakes, also known as multiplying effect brakes, are speciallyequipped with guides that engage the yarn and cause it to follow a pathdeflected laterally from the rectilinear path. For yarn brakingmodulation, these brakes are arranged downstream of means 201 (asindicated at 271A in FIG. 23), along the direction of feed of yarn 31,have movable guides which move with respect to fixed guides, in order toalter lateral deflection of the yarn path and thus the angle at whichthe yarn is wound on the guides, both movable and fixed.

All embodiments described control the tension T of weft yarn duringinsertion in the shed of a loom. FIG. 27 illustrates the pattern of wefttension (curve a) during insertion in a gripper loom, in conventionaltype operation without using the device of the invention.

With bold curve b, FIG. 27 shows the pattern obtained when the device ofthis invention is used. In a conventional type of operation, the cycleof yarn tension T is normally characterised by two distinct parts, whichare a true reflection of the speed pattern (curve f) of the two grippersof the loom, the delivering one and the receiving one. The amount of thefriction encountered by the weft yarn during insertion in the loom shed,and which is responsible to a large extent for its tension, is in factin a first approximation proportional to the speed at which the yarn ispicked by the two grippers. For this reason, in each part of the cyclethere is a maximum tension peak roughly corresponding to the grippermaximum speed condition and, for the same reason, the central part ofthe cycle normally has a point at which tension tends to be null,corresponding to the changeover of yarn between the two grippers, whichtakes place at a yarn speed which is practically null. At the start ofinsertion, there is generally a peak c caused by sharp acceleration ofthe yarn. FIG. 27 shows that the device serves as a means for avoidingthe yarn tension T having a pattern closely linked to speed Vp of thegrippers with resultant peaks. The device maintains tension T very closeto a predetermined tension pattern, such as defined for example by aconstant threshold level TMAX, independently from speed cycle of thegrippers. According to curve d of the current absorbed by the brake andcorresponding to the braking force FB exerted by the latter on the yarn,the brake intervenes repeatedly in each part of the loom cycle tomodulate braking force FB and thus keep tension substantially close tothe defined value TMAX, in spite of variations of the yarn speed. Thebrake intervenes instantly to vary braking force FB each time thethreshold TMAX is crossed through by tension T, whether in the ascendingor descending direction, respectively to decrease or increase brakingforce FB.

At the start of the cycle when the weft yarn is accelerated, the brakeis deactivated almost immediately (part h of curve d), avoiding aninitial tension peak. In the center, when yarn is exchanged between thetwo grippers, the brake tends to remain active for longer (portion e ofcurve d), thus allowing yarn 31 to be under tension during thechangeover. Further, in the time between one insertion cycle C1 and thenext C2, the brake ceases modulation and generates a constant brakingforce (portion g of curve d). During this time, the yarn is motionlessand thus subjected to a static tension of an essentially null orreasonably low value, given that despite the pressing action of thebrake on the yarn, the latter does not slide with respect to the brake.

Similar considerations may also be made with reference to other types oflooms, such as air jet or projectile looms.

With devices 270, 290 and 310, yarn tension during insertion in the loomshed is controlled without imposing any lateral deviations on the yarntrajectory. The brakes have two faces or guides that press on the yarnfrom two opposite sides like a clamp in order to guide the yarn andcontrol its tension during insertion, and that also remain substantiallymotionless during operation, with the result that these guides do notperform transverse movements with respect to the yarn path (fixed guideor clamp brakes).

Brakes referred to earlier altering yarn trajectory laterally by meansof movable guides in order to control yarn tension may be used as analternative to the fixed guide brakes (movable guide or yarn lateraldeflection brakes). These are different from fixed guide brakes. Thesedifferences might result in inferior performance of the movable guidebrakes with respect to the fixed guide brakes in controlling yarntension during insertion in the loom.

If the yarn lateral deflection or movable guide type brakes are used,for example, with all other conditions being equal, a lower yarn tensioncontrol speed is implicitly obtained during insertion, on account of thefar from negligible times needed for the movable guides to move from oneposition to the other in controlling yarn tension. Similar movableguides may consist of retractile guides, each arranged between two fixedguides and controllable in a positive sense to deflect a portion of theyarn sideways with respect to the fixed guides. In this way, it ispossible to alter the angle at which the yarn is wound on the guides,whether fixed or retractile, for varying, as a result, the brakingexerted by the friction of the guides on the portion of yarn andconsequently the yarn tension. Similar movable guides may also consistof guides having a passive operation, such as for example guides thatyield under the yarn tension action. In both cases, the rapidity of thedevice's response in controlling yarn tension is certainly conditionedby the time it takes the movable guides to complete their movements. Nosuch conditioning is imposed by the fixed guide brakes.

The action of movable guide brakes, operating to vary yarn braking byaltering the angle at which the yarn is wound on movable and/or fixedguides, is achieved substantially through a multiplying effect on theyarn tension. These movable guide brakes operate substantially asamplifiers of the tension in the yarn at the brake input, so that yarntension at the brake output depends on the input tension value. If thelatter is null, for example, variation of the winding angle describedearlier does not produce any yarn tension variation effect. Performanceof these brakes may be limited by the fact that a non-null yarn inputtension, better still if controlled, is required.

In fixed guide brakes, the fixed guides are suitable for co-operationwith the yarn in a relationship of pressure. As being in contact withthe yarn and associated with electromagnetic means to vary theirpressure on the yarn, these fixed guides can in fact put the yarn intension without depending on the existence of any yarn input tension, aswell as on its value. The fixed guide brakes also have the advantage ofnot causing any lateral movements of the yarn which could, if rapid,cause corresponding tensions in the yarn because of its lateral inertia.Operation of the clamp brakes that do not laterally alter trajectory ofthe yarn, is substantially exempt of any dynamic phenomena that couldhave a negative effect on speed and precision of the yarn tensioncontrol. With high and rapidly varying insertion speeds, fixed guidebrakes intervene to vary the braking force FB on the yarn during allparts of the insertion cycle, with a promptness and frequency thatensure a practically continuous yarn tension control in time and rapidrestoration of the yarn tension programmed pattern after even theslightest deviation of the yarn tension from it.

A surprising effect obtained from the device (particularly, though notexclusively, when equipped with fixed guide brakes) is that yarn tensionmay be controlled during insertion in a way that is completelyindependent of the loom cycle, i.e. without synchronising with thiscycle and without receiving any commands from the loom cycle controlunit, in virtue of the very high speed and precision with which thedevice intervenes to control tension. Obviously, this does not excludecombinations in which the device of the invention is operable in such away as to interact with the loom cycle control unit for intervening, forexample, at predetermined times during the cycle.

In FIG. 28, balancing current CUR1 supplied to the electromagneticcontrol means 320 in order to keep the relatively movable controlelement 321 (corresponding to rotor 40, armature 64, coil 203, winding252 of the movable coil rotating actuator 251, etc) motionless, isproduced by a driving circuit 322 as a function of an analogic offsetsignal S1. The latter signal can be obtained from comparator circuit 324by detecting the difference between the analogic signal S issued byposition sensor 323 and a constant value reference signal RIF,corresponding to the signal generated by position sensor 323 whenmovable element 321 is exactly in its predetermined position. CurrentCUR1 is controlled by circuit 322 in order to prevent any substantialdisplacement of movable element 321 from its predetermined position andits value is measured by sampling the voltage on the terminals of aresistor R across which current CUR1 flows, using a high impedancemeasuring circuit 326 so as not to significantly alter the value ofCUR1.

FIG. 29 illustrates a comparator circuit 327 for comparing the signal Scoming from position sensor 323 with a sawtooth signal generated by thecircuit 328 and outputting a logic signal S2 that is dependent onoutcome of the comparison. This logic signal S2 is suitable forcontrolling a gate 329 placed between a switching circuit 331 and aclock circuit or time base 332 generating a constant frequency signalCK. Accordingly, gate 329 enables or disables passage of signal CKthrough it and controls activation of switching circuit 331 by signal CKto permit or inhibit a current generator 333 to supply electromagneticmeans 320 with a balancing current CUR2. Finally a count circuit 334determines the percentage of time for which the gate is active andallows passage of signal CK, so that size can be calculated of thebalancing current CUR2 supplying electromagnetic means 320 at any time.

Although particular preferred embodiments of the invention have beendisclosed in detail for illustrative purposes, it will be recognizedthat variations or modifications of the disclosed apparatus, includingthe rearrangement of parts, lie within the scope of the presentinvention.

What is claimed is:
 1. A device for controlling tension of a yarn at theoutlet of a yarn feeder provided with a drum about which a reserve ofyarn windings are disposed for feeding the yarn into a weaving machine,said device comprising:an eyelet disposed coaxially with an axis definedby the drum for guiding the yarn along the drum axis towards the weavingmachine as the yarn is drawn off of the reserve, said eyelet beingmovable along the drum axis and the yarn extending through the eyeletand exerting a first force thereon which is proportional to the yarntension and which is oriented along the drum axis; and a yarn tensionmeasuring arrangement including a control circuit, a position sensor,and an electromagnetic control to which current is supplied by saidcontrol circuit, said electromagnetic control having a movable controlelement operable electromagnetically and connected to said eyelet formovement therewith, said position sensor communicating with said controlcircuit and providing a signal thereto upon displacement of said movablecontrol element from a predetermined measuring position andcorresponding to displacement of said eyelet along the drum axis as aresult of said first force, said control circuit providing saidelectromagnetic control with current in response to said signal fromsaid position sensor such that said electromagnetic control generates asecond force which moves said movable control element to balance saidfirst force and maintain said movable control element substantiallymotionless in said predetermined measuring position during variations inthe yarn tension, said tension measuring arrangement including means formeasuring said current and generating a signal which corresponds to theyarn tension.
 2. The device of claim 1 wherein said electromagneticcontrol includes a magnetic circuit which generates a magnetic field,and said movable control element includes a coil arranged coaxially withthe drum and connected to said eyelet for movement therewith, said coilbeing disposed at least partially in said magnetic field and uponreceiving said current from said control circuit, said coil interactswith said magnetic field and generates said second force to maintainsaid eyelet and said coil substantially motionless during variations inthe yarn tension.
 3. The device of claim 2 further including a supportmember which is fixed relative to the drum and an elongate tubularsleeve connected to said eyelet for movement therewith along the drumaxis and through which the yarn passes, said sleeve being axiallyslidingly supported on said support member and mounting thereon a flangewhich projects radially outwardly therefrom, said coil being connectedto said flange adjacent an outer periphery thereof, and said positionsensor being mounted generally axially between said eyelet and saidflange.
 4. The device of claim 1 further including a support memberwhich is fixed relative to the drum and a ball bearing mounted on saidsupport member, said eyelet having a portion which is slidably mountedin said ball bearing to permit axial sliding movement of said eyeletrelative to said support member.
 5. The device of claim 1 furtherincluding a support member which is fixed relative to the drum and atleast one elastic support element mounted on said support member andassociated with said eyelet to permit axial sliding movement of saideyelet along the drum axis relative to said support member.
 6. Thedevice of claim 1 wherein said electromagnetic control includes anelectromagnet and a spring device, said movable control element includesan armature connected to said eyelet for movement therewith, said springdevice being disposed to exert a control force on said armature whichcooperates with said second force and acts in a direction oppositethereto to maintain said armature and said eyelet substantiallymotionless in said predetermined measuring position during variations inyarn tension.
 7. The device of claim 6 further including a supportmember which is fixed relative to the drum, said electromagnet includinga metallic casing mounted on said support member and a winding disposedwithin said casing, said casing slidingly supporting said armaturetherein radially inwardly of said winding and including a flange whichprojects radially inwardly toward the drum axis, said flange having afirst side which faces said armature and a second side which faces awayfrom said first side, said spring device including a first springdisposed between said first side of said flange and said armature and asecond spring disposed between said second side of said flange and aradially projecting part of said eyelet.
 8. The device of claim 7further including an elongate tubular sleeve connected to said eyeletfor movement therewith, said armature having a tubular shape andmounting coaxially therein said sleeve such that said eyelet, saidsleeve and said armature move with one another along the drum axis, saidarmature having a first terminal end adjacent said first spring and asecond terminal end axially spaced from said first terminal end, saidposition sensor being mounted on said casing adjacent said secondterminal end of said armature.
 9. The device of claim 1 furtherincluding a support member which is fixed relative to the drum, saidelectromagnetic control includes a motor having a fixed housingsupported on said support member, a rotor and a shaft projecting fromsaid housing and connected to said rotor for rotation therewith, saidmovable control element comprising said rotor, said electromagneticcontrol further including an arm having one end fixed to said shaft andan opposite end fixed to and supporting said eyelet, said first forcesubjecting said shaft to a torque via said eyelet and said arm, and saidcurrent is supplied by said control circuit to said motor such that saidmotor generates said second force to balance said torque and maintainsaid rotor substantially motionless in said predetermined measuringposition during variations in the yarn tension.
 10. The device of claim9 wherein a blade is fixed to said shaft for movement therewith and saidposition sensor is supported on said support member adjacent said bladeto provide said signal to said control circuit upon movement of saidrotor from said predetermined measuring position.
 11. The device ofclaim 1 further including a support member which is fixed relative tothe drum, said electromagnetic control includes at least one permanentmagnet fixed to said support member, and said movable control elementincludes a winding and a lever pivotably mounted on a fulcrum fixed tosaid support member and defining an axis perpendicular to the drum axis,said winding being associated with one end of said lever for movementtherewith and being movable parallel to and at a constant distance fromsaid permanent magnet, an opposite end of said lever being fixed to andsupporting said eyelet, said lever transmitting to said winding adisplacement of said eyelet as a result of said first force, and therelationship between the displacement of said eyelet and thecorresponding displacement of said winding relative to said permanentmagnet is defined by the relationship between the respective distancesof said eyelet and said winding from said fulcrum.
 12. The device ofclaim 11 further including a casing attached to said support member, twoof said permanent magnets are supported on said casing in a spaced apartmanner from one another and said winding is disposed therebetween formovement parallel to and at a constant distance from each of saidpermanent magnets.
 13. The device of claim 1 further including a brakehaving at least one movable guide element which contacts the yarn anddeflects same transversely with respect to the path of the yarn alongthe drum axis in response to said signal to modify yarn braking, saidbrake being arranged downstream of said tension measuring arrangement inthe yarn feed direction.
 14. The device of claim 1 further including abrake assembly disposed between the drum and said tension measuringarrangement to vary braking of the yarn based upon said signal, saidbrake assembly including a pair of opposed discs which are arrangedcoaxially with respect to the drum axis and between which the yarnpasses as same unwinds from a terminal end of the drum, and a magneticcircuit which magnetically varies the braking force on the yarn passingbetween said discs based upon said signal, one of said discs beingdisposed adjacent said tension measuring arrangement and beingring-shaped so as to define a central hole through which the yarn passesas same approaches said tension measuring arrangement, said eyelet beingdisposed inside said hole of said one disc to receive the yarn frombetween said discs.
 15. The device of claim 14 further including a coremember mounted on the terminal end of the drum and the other said discis mounted on said core member such that a braking surface thereof isjuxtaposed with a braking surface defined on said one disc, said coremember including a coil disposed adjacent a side of said other discopposite said braking surface thereof, and said coil, said core and saidone disc forming said magnetic circuit.
 16. The device of claim 15further including a support member which is fixed relative to the drumand an elongate tubular sleeve connected to said eyelet for movementtherewith along the drum axis and through which the yarn passes frombetween said discs, said sleeve being axially slidingly supported onsaid support member and mounting thereon a flange which projectsradially outwardly therefrom, said coil being connected to said flangeadjacent an outer periphery thereof, and said position sensor beingmounted generally axially between said eyelet and said flange.
 17. Thedevice of claim 14 wherein said brake assembly includes a springdisposed to bias said discs toward one another, and said magneticcircuit generates a force which opposes a biasing force of said springto move the discs away from one another to reduce braking on the yarn,said spring being adjustable so as to selectively vary the biasing forceapplied to said discs.
 18. The device of claim 17 further including asupport member which is fixed relative to the drum, the other said discbeing mounted on the terminal end of the drum such that a brakingsurface thereof is juxtaposed with a braking surface defined on said onedisc, said spring being disposed between a side of said one discopposite said braking surface thereof and an axially adjustable nutassociated with said support member, said other disc being fixed andsaid one disc being movable relative thereto, said spring biasing saidone disc toward said other disc.
 19. The device of claim 1 furtherincluding a brake assembly disposed between the drum and said tensionmeasuring arrangement, said brake assembly including a ring-shaped brakearranged coaxially with the drum and biased toward a braking surfacedefined circumferentially along the drum to brake the yarn as sameunwinds from the drum and passes between the brake and the brakingsurface, said eyelet being disposed to receive and engage the yarn frombetween said brake and the braking surface, said brake assemblyincluding an electromagnetic control device associated with said brakewhich is selectively driven by said tension measuring arrangement as afunction of said current to actuate said brake and vary braking on theyarn to maintain the yarn tension in conformance with a predeterminedpattern.
 20. The device of claim 19 wherein the braking surface has arounded shape and defines a terminal end of the drum from which the yarnis withdrawn, and said brake is a ring-shaped truncated cone which isaxially biased against the rounded braking surface.
 21. The device ofclaim 19 wherein said brake assembly includes a resilient member whichbiases said brake against the drum, said electromagnetic control deviceincluding a magnet disposed to attract said brake in opposition to thebiasing force of said resilient member in order to reduce braking on theyarn.
 22. The device of claim 19 further including a support memberfixed relative to the drum, said magnet is ring-shaped and is connectedto said support member so as to substantially surround said eyelet, saidmagnet including a core having a coil therewithin, and a flange projectsradially outwardly from one end of said brake adjacent said magnet andis attracted thereto, said current provided to said electromagneticcontrol is a first current, and said control circuit provides a secondcurrent to said magnet based upon said first current to vary theattraction force between said magnet and said flange to modulate brakingof the yarn.
 23. The device of claim 1 further including a displaydevice configured to display a value corresponding to instantaneous yarntension based upon said signal.
 24. The device of claim 1 furtherincluding a brake assembly disposed adjacent said tension measuringarrangement and a braking circuit which controls said brake foroperation in either a constant braking mode or a modulated braking modebased upon said current.
 25. A method of measuring the tension of yarnat the outlet of a yarn feeder provided with a drum about which areserve of yarn windings are disposed for feeding into a weavingmachine, said method comprising:providing an eyelet coaxial with an axisdefined by the drum and an electromagnetic control having a movablecontrol element connected to the eyelet for movement therewith; drawingthe yarn from the drum and through the eyelet and guiding the yarn alongthe drum axis towards the weaving machine whereby the yarn exerts afirst force on the eyelet upon passing therethrough which isproportional to the yarn tension; displacing the eyelet and the movablecontrol element along the drum axis from a predetermined measuringposition with said first force; sensing a displacement of the movablecontrol element from said predetermined measuring position andsignalling a control circuit; supplying the electromagnetic control withcurrent from the control circuit and generating a second force to movethe movable control element in opposition to each displacement thereoffrom said predetermined measuring position as a result of said firstforce to maintain the movable control element substantially motionlessin said predetermined measuring position during variations in yarntension; and detecting the value of said current and generating a signalwhich corresponds to the yarn tension.
 26. The method of claim 25including providing a brake and controlling the brake so as tocontinuously adapt braking of the yarn during a cycle of insertion ofthe yarn into the weaving machine to avoid peaks in yarn tension and tomaintain conformance of the yarn tension with a predetermined yarntension pattern.
 27. The method of claim 26 including driving the brakesynchronously with respect to the insertion cycle such that braking ofthe yarn is effected exclusively as a function of the yarn tension basedupon said current.
 28. The method of claim 26 including providing amicrocontroller which stores a predetermined yarn tension value andreceives a braking signal based upon said current and corresponding toinstantaneous yarn tension, operating the brake in either a first modewherein the brake exerts a constant braking force on the yarn or asecond mode wherein the brake exerts a variable braking force on theyarn, and when the brake is operated in said second mode: increasing thebraking force of the brake when the signal corresponding to yarn tensionis lower than the predetermined yarn tension value; or decreasing thebraking force of the brake when the signal corresponding to yarn tensionis greater than the predetermined yarn tension value.